.Net Standard is a specification of the .NET API's that are available on all implementations of .NET.
What does this mean? - Calling an API in .Net Standard will run on all .Net versions that have implemented that version of the Standard.
SO what does that mean? - .Net Core introduces cross platform development. The need to now possibly share code between a Windows machine and a Linux box require a "standard" that can be implemented across all equally.
Think of it as a contract. If a version of .Net implements .Net Standard 2.0, it is saying that it will have a version of the API's in that standard available for use on all platforms.
So, back to your original question. .Net Standard 2.0 is not a "new" version of the framework. It is the contract that your subproject, assuming it's a class library is going to abide by. If an API appears to be missing it is because it doesn't have it as part of it's contract (maybe). I say maybe because it's not the easiest to find.
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Here is a table of the versions of .Net that support the versions of .Net Standard.
In your case 4.6.1 doesn't even support 2.0. The last version of the Standard that 4.6.1 supported is 1.4.
I thought 4.6.1 supported .Net standard 2.0 after I wrote the above and upon further reading found a more up to date chart along with this post stating it is supported in 4.6.1. sigh
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This SO answer seems to indicate that 1.4 does have Resources in it but it was preview versions and things change. I don't see it in the APIs supported list but I could have missed it. The SO answer seems to indicate it was a tooling issue and could be worked around. That could be fixed by now....
Here's where you can look to see what each Standard supports for APIs.
HOWEVER, I don't think this is actually what you want.
I think what you really want is to simply change your sub project's target to 4.6.1 as is your main app since I highly suspect you have no need/desire to run cross platform.
Additionally, prior to .Net Standard 2.0, thousands of API's were not in the Standard. IMO, prior to 2.0, the Standard is a hard sell especially for users on the .Net Full Framework.